High temperature fuel cells such as solid oxide fuel cells comprise an electrolyte sandwiched between a cathode and an anode. Oxygen reacts with electrons at the cathode to form oxygen ions, which are conducted through the ion-conducting ceramic electrolyte to the anode according to the reaction:½O2+2e→O2−  (1)
At the anode, oxygen ions combine with hydrogen and carbon monoxide to form water and carbon dioxide thereby liberating electrons according to the exothermic reactions:H2+O2−→H2O+2e−  (2)CO+O2−→CO2+2e−  (3)
In conventionally-designed solid oxide fuel cells, the above electrochemical reactions usually are performed at temperatures of between 600° C. and 1000° C. Therefore, thermal management is an important consideration in the design of fuel cell systems. SOFC stacks produce high grade heat and it would obviously improve the overall efficiency of the operation if that high grade heat could be captured and utilized.
Typically, incoming fuel and air streams are preheated both during startup when the stack is at ambient temperatures and during operating conditions when the stack is at elevated temperatures. It is well known to use heat exchangers to extract heat from the stack exhausts and use that heat to preheat incoming gas streams.
In PCT Application No. PCT/US02/12315 (WO02/087052), a waste energy subassembly is provided which includes a combustion zone and a heat exchanger. A separate reformer subassembly provides reformate to the combustion zone where it is combusted to heat the system. Once at operating conditions, the stack exhaust is combusted in the combustion zone and heat is transferred to the incoming air and reformate streams in the heat exchanger. In Applicant's co-pending PCT Application No. CA01/01014, an integrated module is described which is associated with a fuel cell stack and includes an afterburner, a fuel reformer and a heat exchanger. The afterburner burns unused fuel in the fuel cell exhaust streams and heats the fuel reformer and an incoming cathode air stream.
It is a goal of both of these technologies to thermally integrate the fuel cell system and some thermal integration is achieved. However, it is apparent that further integration and better efficiencies may be achievable.
Therefore, there is a need in the art for a highly thermally integrated fuel cell system.